Stability Analysis of 6MW Wind Turbine High Speed Coupling … · 2017-10-02 · Also, the high...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 18 (2017) pp. 7470-7477

© Research India Publications. http://www.ripublication.com

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Stability Analysis of 6MW Wind Turbine High Speed Coupling using the

Finite Element Method

Hanyong On1 , Junwoo Bae1, JongHun Kang2, HyoungWoo Lee2 , Seungkeun Jeong3 and SooKeun Park4#

1Department of Integrative Engineering graduate, Jungwon University, Chungbuk,, South Korea.

2Department of Mechatronics Engineering, Jungwon University, Chungbuk,, South Korea. 3JAC Coupling Co., Ltd., Busan, South Korea.

4Korea Institute of Industrial Technology, Incheon, South Korea. #Corresponding author

Abstract

The objective of this study was to design a high speed

coupling for large scale 6MW wind turbines and assess its

structural stability. A high speed coupling requires both high

stiffness and flexibility since axial displacement and lateral

displacement need to be absorbed while transferring power at

high speeds.

Also, the high speed coupling must ensure a service life of 20

years, necessitating high structural safety and fatigue

resistance. Using the wind turbine design data, a finite

element analysis was carried out on the coupling based on the

maximum torque transfer and allowable displacement, to

assess its structural safety. Afterwards, the SN curves for each

material were obtained to evaluate the durability performance

by calculating the damage to each component, using the

Markov Matrix or fatigue data for the turbine according to the

Palmgren Miner Rule. Accelerated fatigue testing was

performed to verify the calculation results, and the analytical

feasibility was verified.

Keywords: Wind Turbine, Finite Element Analysis, Fatigue

Analysis, SN Curve, High Speed Coupling

INTRODUCTION

The high speed coupling of a wind turbine not only requires

the characteristics of a generic industrial coupling, which

transfers power while absorbing axial and radial

displacements, but also needs to be a functional electrical

insulator to prevent the flow of high current to the gearbox.

[1] The high speed coupling is a key component of wind

turbines. Its design and the verification of its functionality and

components are detailed in the IEC61400 and GL Guidelines.

For use in wind turbines, the high speed coupling component

needs to have a service life of at least 20 years under

conditions where the axial distance between the gearbox and

the generator varies, and axial misalignment occurs. [2],[3]

The magnitude of axial misalignment for the wind turbine

coupling is very large compared to the axial displacement of a

generic disc coupling, when considering the level of torque.

This makes it necessary to conduct a structural safety analysis

of the coupling’s composite materials, flexibility and

insulating characteristics, while compensating for the large

displacement and guaranteeing the service life of the coupling.

[4],[5] Additionally, fatigue analysis and experiments are

necessary to validate the 20 year service life.

The stress and safety factors applied for the maximum

displacement of industrial couplings, including wind turbine

couplings, lie within the category of general structural

mechanics, and are generally applied to representative

couplings, including disc couplings. [6]~[8]

In this study, the fatigue life calculation for the designed wind

turbine coupling was carried out by using the Damage

Accumulation Model with the Palmgren Miner Rule, which

uses the SN curve and the stress amplitude that occurs when a

fatigue load is applied. [9]~[12]

In order to verify the feasibility of the structural safety and

fatigue analyses, fracture testing and accelerated fatigue

testing were performed at peak torque, and the safety of the

developed product was determined.

HIGH SPEED COUPLING STRUCTURE AND

FUNCTION

The high speed coupling is a functional part of the wind

turbine generator powertrain, and is located between the

gearbox and generator in the wind turbine power generator

system. The high speed coupling delivers high speed torque

from the gearbox to the generator. Figure 1 shows the high

speed coupling structure. The major structural components in

the high speed coupling include a flexible disc pack, glass

fiber reinforced composite spacer, a hub set with tapered

fitting, and a torque limiter. The flexible disc pack absorbs

displacements from axial, radial, and angular misalignments

in the operating environment of the wind turbine generation

system, and delivers rated power. The glass fiber reinforced

composite spacer structure performs as insulation to prevent

electrical damage to the gearbox which can occur with current

backflow from the generator. Also, the tapered fitting hub set



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connects the gearbox and generator axis to the coupling, while

the torque limiter cuts off power when overloading is detected.

Figure 1: High speed coupling structure

HIGH SPEED COUPLING STRUCTURAL ANALYSIS

In this study, a finite element analysis was carried out to

assess the structural stability of the high speed coupling

structure. First, the 3-dimensional design program CATIA V5

was used to model each part, and then a structural analysis

was conducted using the finite element analysis software

ANSYS V13. The load conditions applied included the torque,

axial misalignment, and radial misalignment. Also, the

combined load of these loads was applied, and the stability of

the final structure and fatigue stability were evaluated.

Figure 2 shows the finite element model of the 6MW high

speed coupling. The basic structure of the coupling consists of

a disc pack, composite spacer, spacer flange, hub set, torque

limiter, and brake disc.

Figure 2: 6MW high speed coupling finite element model

Table 1 shows the material properties of the SPS6 used for the

high speed coupling disc.

Table 1: SPS6 material properties

Symbol SPS6

Young’s modulus(GPa) E 210

Poisson’s ratio υ 0.3

Yield strength(MPa) Y 1,078

Tensile strength(MPa) Ut 1,226

The composite material used for the spacer was filament

wound glass fiber reinforced composite with a winding angle

of [±55].

Equivalent properties were used to apply the material

properties of the filament wound composite for structural

analysis. The equivalent properties of the composite material

were obtained using the preprocessing functionality of

MSC.Patran. The composite material properties obtained

experimentally were inputted. Figure 3 shows the MSC.Patran

input window.

Figure 3: Input of Composite Properties

As shown in Figure 4, the thickness and alternating

orientation [±55] of the stacked fiber layers were used to input

the material properties according to the stacking angle.

Figure 4: Filament winding composite stacking sequence

The Young's modulus and Poisson's ratio were determined

according to composite theory. The results are shown in Fig. 5.



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Figure 5: Equivalent material properties of the filament

winding composite [±55]

Table 2 shows the final equivalent material properties of the

filament winding composite.

Table 2: Equivalent material properties of the filament

winding composite[±55]

Material property Symbol Unit Value

Young’s modulus E1 GPa 12.1

E2 GPa 20.2

E3 GPa 12.1

Poisson’s ratio v12 0.388

v23 0.492

v13 0.292

Shear modulus G12 GPa 14.5

G23 GPa 4.65

G13 GPa 4.65

Table 3 and Fig. 5 show the boundary conditions for the

structural analysis of the 6MW wind turbine high speed

coupling. The first condition was a maximum torque of

120,000 Nm, the second load condition was an axial

misalignment of 10 mm, and the third load condition was a

radial misalignment of 25 mm. The combined load was

determined used the load integration feature of ANSYS and

the three individual load conditions. Table 4 shows the input

values used in the structural analysis.

Table 3: Boundary conditions for the structural analysis

Case Load / Displacement Values

1 Torque 120,000

[Nm]

2 Axial Misalignment 10[mm]

3 Angular Misalignment 25[mm]

4 Combination Load 1+2

5 Combination Load 1+3

6 Combination Load 1+2+3

(a) Torque boundary condition

(b) Axial misalignment boundary condition

(c) Angular misalignment boundary condition

Figure 5: Boundary conditions of the high speed coupling

Table 4: Input value for FE Analysis

Part Material Young’s

modulus

[Gpa]

Poisson’s

ratio

Yield

Strength

[Mpa]

Tensile

strength

[Mpa]

Disc

Pack

SPS6 210 0.3 1,078 1,225

Brake

Disc

SM490A 210 0.3 345 570

Flange SCM440 210 0.3 835 930

GFRP

Spacer

Filament

winding

33.12 0.28 - 248

Tables 5 ~ 6 and Figures 6 ~ 8 show the structural analysis

results of the 6MW high speed coupling. The full model

structural analysis results for each load condition of the 6MW

high speed coupling revealed a safety factor of 2 or greater

based on the yield strength compared to the maximum stress

for all the major components including the brake disc, flange,



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spacer, and torque limiter. The safety factor was found to be

1.5 for the disc pack. Also, the safety of the design, which was

verified to be the safety factor for all the major parts, was 1.5

or greater when the load conditions in the axial, angular, and

radial directions were combined.

Table 5: Maximum Stress unit: [Mpa]

Case C1 C2 C3 C4 C5 C6

Value Torque

120kNm

Axial

10 mm

Angular

25 mm C1+C2 C1+C3 C1+C2+C3

Brake Disc 152.9 1.8 8.7 152.9 152.8 152.8

Disc Pack 511.5 190.6 141.7 604.9 568.4 737.6

Flange A 416.1 2.5 66.0 415.7 439.6 439.2

Spacer 119.7 0.1 19.5 119.7 119.9 119.9

Flange B 357.4 2.3 119.7 357.5 360.7 360.8

Torque

Limiter A 356.8 2.5 119.9 357.4 355.8 356.4

Torque

Limiter C 124.5 0.3 119.9 139.9 139.3 139.2

Table 6: Safety Factor

Case C1 C2 C3 C4 C5 C6

Value Torque

120kNm

Axial

10 mm

Angular

25 mm

C1+C2 C1+C3 C1+C2+C3

Brake Disc 2.3 191.7 39.7 2.3 2.3 2.3

Disc Pack 2.1 5.7 7.6 1.8 1.9 1.5

Flange A 2 334 12.7 2 1.9 1.9

Spacer 2.1 2,480 12.7 2.1 2.1 2.1

Flange B 2.3 363 10.3 2.3 2.3 2.3

Torque Limiter A 2.3 334 238.6 2.3 2.3 2.3

Torque Limiter C 6.7 2,783.30 759.1 6 6 6

(a) brake disk (b) disk pack

(c) flange A (d) spacer

(e) flange B (f) torque limiter A

(g) torque limiter C

Figure 6: 6MW high speed coupling structural analysis

(Torque = 120,000 Nm)




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\


(e) flange B (f) torque limiter

A



(Axial Misalignment = 10 mm



(e) flange B (f) torque limiter A



(Angular Misalignment = 25 mm)

HIGH SPEED COUPLING COMPOSITE FATIGUE

TESTING

The high speed coupling employs steel materials and

composite materials for insulation. The SN curve of the steel

materials has been widely studied and reported in the

literature. However, the properties of composites differ

depending on the winding angle, so the SN curve was

determined experimentally. The composite component that

delivers power has a winding angle of 55 degrees, and so the

specimens used for fatigue testing were fabricated with an

orientation of 55 degrees using a plate. Figure 9 shows the

fabricated specimens and the image of a fractured specimen

following testing.



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Figure 9: Composite specimens and test specimen

The fatigue testing was carried out with a stress ratio of R=0.1

and 4 load steps were conducted. A minimum of 3 specimens

were used for each step. Figure 10 shows the SN curve of the

high speed coupling composite as determined by fatigue

testing.

Figure 10: SN Curve of GFRP

Fracture strength testing is necessary for composite materials.

This was performed using tensile testing. Tensile tests of 5

specimens were performed to obtain the average tensile

strength of 248.5Mpa. The test results are shown in Table 7.

Table 7: GFRP tensile test results

No. Width

[mm]

Thickness

[mm]

Area

[mm2]

Force

[N]

Strength

[MPa]

1 23.82 3.50 83.37 20.85 250.1

2 23.66 3.37 79.73 19.83 248.7

3 23.81 3.37 80.24 19.94 248.5

4 23.82 3.25 77.42 19.17 247.6

5 23.65 3.28 77.57 19.21 247.7

The fatigue properties of the SCM440 material referenced in

the literature, and its SN curve, are shown in Fig. 11. [13]

Figure 11: SN Curve of SCM440

HIGH SPEED COUPLING FATIGUE ANALYSIS

For the fatigue strength analysis, the Markov Matrix, which

considers the torque and count applied to the high speed

coupling and the maximum stress of each part when torque is

applied, were calculated to determine the coupling’s safety

using the Accumulated Damage of the Palmgren Miner Rule.

Figure 12 shows the Markov Matrix, which is the design load

data for the 6MW wind turbine.

Figure 12: Markov Matrix curve of 6MW Turbine

The Accumulated Damage value was calculated using Eq. (1)

which utilizes the Palmgren Miner Rule using the Markov

Matrix, involving the load and cycle data.

1Ri

Ei

NnD (1)

Here, Ein is the number of load cycles within the stress range

of one category and RiN is the number of allowable load

cycles within the stress range of one category.



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Table 8 shows the Accumulated Damage values for the steel

material Flange A and composite tube which were determined

using the SN curves of Figs. 10 and 11 and Eq. (1). The

Accumulated Damages were all below 1, verifying the safety

of the parts.

Table 8: Accumulated Damage

Parts Flange A Composite Tube

Accumulated Damage 0.0483 0.9211

HIGH SPEED COUPLING TESTING

For the fabricated high speed coupling, damage was evaluated

when the load condition of Case 3 was applied, and fatigue

testing for the Markov Matrix was conducted. The damage for

Flange A was set to be greater than 1 to minimize the number

of test cycles. Table 9 shows the test conditions and the

damage values for Flange A and the composite tube.

Table 9: Conditions and damage values for the accelerated

fatigue testing

Torque

[Nm] Test Cycle Flange A Composite

57,000 1,000,000 0.49366 0.008484

65,000 10,000 0.29369 0.002588

120,000 400 0.26721 0.000176

Summary 1.0546 0.0112

The calculation results shown in Table 9 reveal that

accelerated testing with heavy loads is possible for steel

materials, but it can be observed that the accumulation of

cycles dominates the fatigue life of composite materials,

rather than the load magnitude. Figure 13 shows the

accelerated test conditions for the high speed coupling.

Figure 13: 6MW High Speed Coupling Fatigue Test

CONCLUSION

Finite element analysis, fatigue analysis, and fatigue testing

were carried out for each component to design a 6MW wind

turbine high speed coupling, and the following conclusions

were obtained in this study.

1) A maximum torque of 120,000 Nm, an axial misalignment

of 10 mm and a radial misalignment of 25 mm were applied to

the structural analysis of a 6MW wind turbine high speed

coupling. Also, the combined load of the three load conditions

was applied to assess the structural stability.

2) Structural analysis showed that the safety factor based on

the yield strength compared to the maximum stress for all the

major components including the brake disc, flange, spacer,

torque limiter, and disc pack was 1.5 or greater. Thus, the

safety of all the designs was verified.

3) The high speed coupling employs steel materials and

composite materials for insulation. Since the composite

material exhibits different properties depending on the

winding angle, the SN curve was obtained through

experimentation.

4) For the fatigue analysis, the Accumulated Damage value

was determined through the Palmgren Miner Rule using the

Markov Matrix, which includes the load and cycle data.

Safety was verified, since the Accumulated Damage for all

parts was less than 1.

5) Although accelerated testing with heavy loads is possible

for steel materials, it was found that for composite materials,

cycle accumulation dominated fatigue life, rather than the load

magnitude.

ACKNOWLEDGEMENT

This study was performed as part of the "The development of

6MW class high speed shaft coupling for offshore wind

turbine over 120kNm maximum torque" under the Energy

Technology Development Project (20143030021090).

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[1] "Power transmission engineering, Flexible shaft

couplings - Parameters and design principles", 1975,

DIN740 Part 2.

[2] J.H Kang etc., 2014, "Development of high speed

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Korean Society of Marine Engineering, Vol. 38, No. 3,

pp. 262~268

[3] H.W Lee, J.Y Han and J.H Kang, 2016, " A study on

high speed coupling design for wind turbine using a

finite element analysis" Journal of Mechanical Science

and Technology, Vol.30, No.8, pp. 3713~3718



7477

[4] "Wind turbines - Part 1: Design requirements", 2005,

IEC61400-1

[5] "Guideline for the Certification of Wind turbines", 2010,

Germanischer Lloyd

[6] B.J Hamrock and B.O Jacobson, 1999, "Fundamentals of

Machine Elements", 3rd Edition, McGraw-Hill. \

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[9] “Design of Steel Structures - Part 1-9 : Fatigue", 2005,

EN 1993-1-9

[10] "Guide for the Fatigue Assessment of Offshore

Structures", 2003, ABS

[11] E.K Gamstedt and S.I Anderson, 2001, " Fatigue

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[12] T.Nguyen, H. Tang, T. Chuang, J. Chin, F.Wu and

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[13] S.H Ahn, K.W Nam, I.T Kim, M.Y Lee and D.K Kim,

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Frequency Induction Hardening of SCM440 Steel",

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